The present invention relates to a method of forming a semiconductor device, and more particularly to a method of forming an isolation layer of a semiconductor device in which a fluorine (F) ion implantation process is performed in order to protect trench sides, prevent boron (B) migration and reduce leakage current.
In recent years, when constructing highly-integrated flash memory devices, an isolation layer is formed by a Self-Aligned Shallow Trench Isolation (SA-STI) scheme through which device structures can be formed conveniently, preventing damage to the tunnel oxide layer, and device characteristics can be obtained easily, thus improving device characteristics.
Generally, in NAND flash memory devices, after a trench is formed in a semiconductor substrate, an insulating layer is formed within the trench in order to gap-fill the trench. A High Density Plasma (HDP) oxide layer (i.e., an insulating layer) is largely used as a gap-fill material for gap-filling the trench.
However, as devices shrink, voids and/or seams are generated within the trench due to the HDP oxide layer. To solve the problem, polysilazane (PSZ) (i.e., a spin-on dielectric (SOD) material) is used when the trench is gap-filled.
However, if PSZ is used as the trench gap-fill material, the sides of the trench can become over stressed due to the tensile force caused by the thermal-mechanical properties of the material. The stress also causes the boron (B) dopant to migrate away from the trench, which results in the loss of boron (B) dopant implanted into the sides of the trench.
Furthermore, the boron (B) dopant is depleted from the sides of the trench because of the trench formation process. Accordingly, leakage current is increased in the HVNMOS transistor.
FIGS. 1A and 1B are views illustrating the flow of stress and boron (B) when a trench is filled with a HDP oxide layer or PSZ.
From FIG. 1A, it can be seen that tensile stress is generated in the trench when the trench is gap-filled with PSZ and the boron (B) moves away from the trench. From FIG. 1B, it can be seen that compressive stress is generated in the trench when the trench is gap-filled with the HDP oxide layer and the boron (B) moves towards the trench.
FIG. 2A is a graph showing the concentration of boron (B) verses the depth from the trench surface after an annealing process is performed. The concentration is measured using Secondary Ion Mass Spectroscopy (SIMS).
Referring to FIG. 2A, the curve “a” is when only arsenic (As) is implanted into the trench without an annealing process. The curve “b” is when a trench gap-fill process and an annealing process employing the HDP oxide layer are performed after an ion implantation process is performed on the trench. The curve “c” is when the trench gap-fill process and the annealing process employing PSZ are performed after the ion implantation process is performed on the trench. The curve “d” is when an annealing process for activating implanted ions is performed after the ion implantation process is performed to form a well. The X-axis corresponds to the depth, and the Y-axis corresponds to the concentration.
The curves “b” and “c” have the same process conditions, but differ in the materials used for gap-filling the trench. The curves “a” and “d” are provided to compare the curves “b” and “c”. The curve “d” has a higher degree of boron (B) activation than the remaining curves because the annealing process is performed at a temperature of 100 degrees Celsius.
Comparing the curves “b” and “c”, the curve b has a higher concentration of boron (B) on the trench surface than the curve “c”. As the depth deepens, the trend is reversed, so that the curve “c” is higher in concentration than the curve “b”. It can be seen that the curve “c” using PSZ has a higher concentration of boron (B) on the trench surface than the curve “b” using the HDP oxide layer.
FIG. 2B is a graph showing the concentration of boron (B) verses depth from the trench surface after an annealing process is performed. The concentration is measured using Spreading Resistance Probe (SRP).
Referring to FIG. 2B, the curve “e” is when a trench gap-fill process and an annealing process employing the HDP oxide layer are performed after an ion implantation process is performed on the trench. The curve “f” is when the trench gap-fill process and the annealing process employing PSZ are performed after the ion implantation process is performed on the trench. The curve “g” is when an annealing process for activating implanted ions is performed after the ion implantation process is performed to form a well.
Accordingly, it can be seen that the concentration of boron (B) on the trench surface is higher in the curve “e” using HDP than in the curve “f” using the PSZ oxide layer.
It can also be seen that the graph of FIG. 2B shows the concentration change of boron (B) more clearly compared with the graph of FIG. 2A, and has a similar result as that of FIG. 2A.